# Differentials on quotient CDGAs

The following code throws a `ValueError("The given dictionary does not determine a valid differential")`

:

```
A.<x, y, z> = GradedCommutativeAlgebra(QQ, degrees=(2, 3, 4))
AQ = A.quotient(A.ideal(x^3))
E = AQ.cdg_algebra({y: x^2, z: x*y})
```

It seems the cause is a check `if not self(self(i)).is_zero()`

. Since the error disappears on omitting the `z = x*y`

differential, I presume it's evaluating $d(xy) = x^3$ to be something non-zero, maybe because it is being evaluated in `A`

and not `AQ`

. Is this a bug or am I supposed to define this CDGA in a different way?

`graded_commutative_algebra`

or`GradedCommutativeAlgebra`

is too long to be a tag, and I couldn't find an alternative. Please feel free to retag.This works fine for me in sage 10.2.beta4. Which version of sage do you use ?

also works fine in sage 10.1 as available on https://sagecell.sagemath.org/

@FredericC 9.5-6 from the Ubuntu repositories, running on WSL.

I can confirm that the code works fine on sage 10.1 built from source